Operating Systems:
Internals and Design Principles, 6/E
William Stallings
Chapter 9
Uniprocessor Scheduling
Patricia Roy
Manatee Community College, Venice, FL
©2008, Prentice Hall
Aim of Scheduling
• Assign processes to be executed by the
processor(s)
• Response time
• Throughput
• Processor efficiency
Types of Scheduling
Scheduling and Process State
Transitions
Levels of Scheduling
Long-Term Scheduling
• Determines which programs are admitted
to the system for processing
• Controls the degree of multiprogramming
• More processes, smaller percentage of
time each process is executed
Medium-Term Scheduling
• Part of the swapping function
• Based on the need to manage the degree
of multiprogramming
Short-Term Scheduling
• Known as the dispatcher
• Executes most frequently
• Invoked when an event occurs
– Clock interrupts
– I/O interrupts
– Operating system calls
– Signals
Short-Term Scheduling Criteria
• User-oriented
– Response Time
• Elapsed time between the submission of a request
until there is output.
• System-oriented
– Effective and efficient utilization of the
processor
Short-Term Scheduling Criteria
• Performance-related
– Quantitative
– Measurable such as response time and
throughput
Scheduling Criteria
Scheduling Criteria
Queuing Diagram
Priorities
• Scheduler will always choose a process of
higher priority over one of lower priority
• Use multiple ready queues to
representmultiiple levels of priority
• Lower-priority may suffer starvation
– Allow a process to change its priority based
on its age or execution history
Priority Queuing
Decision Mode
• Nonpreemptive
– Once a process is in the running state, it will
continue until it terminates or blocks itself for
I/O
Decision Mode
• Preemptive
– Currently running process may be interrupted
and moved to the Ready state by the
operating system
– Allows for better service since any one
process cannot monopolize the processor for
very long
Process Scheduling Example
First-Come-First-Served
• Each process joins the Ready queue
• When the current process ceases to
execute, the longest process in the Ready
queue is selected
First-Come-First-Served
• A short process may have to wait a very
long time before it can execute
• Favors CPU-bound processes
– I/O processes have to wait until CPU-bound
process completes
Round Robin
• Uses preemption based on a clock
Round Robin
• Clock interrupt is generated at periodic
intervals
• When an interrupt occurs, the currently
running process is placed in the ready
queue
– Next ready job is selected
• Known as time slicing
Effect of Size of Preemption
Time Quantum
Effect of Size of Preemption
Time Quantum
Queuing Diagram
Shortest Process Next
• Nonpreemptive policy
• Process with shortest expected processing
time is selected next
• Short process jumps ahead of longer
processes
Shortest Process Next
• Predictability of longer processes is
reduced
• If estimated time for process not correct,
the operating system may abort it
• Possibility of starvation for longer
processes
Exponential Smoothing
Coefficients
Use Of Exponential Averaging
Use Of Exponential Averaging
Shortest Remaining Time
• Preemptive version of shortest process
next policy
• Must estimate processing time
Highest Response Ratio Next
• Choose next process with the greatest
ratio
Feedback
• Penalize jobs that have been running
longer
• Don’t know remaining time process needs
to execute
Feedback Scheduling
Scheduling Policies
Scheduling Policies
Comparison of Scheduling
Policies
Formulas
Normalized Response Time
Normalized Response Time
Normalized Response Time
Normalized Turnaround Time
Simulation Result for Waiting
Time
Fair-Share Scheduling
• User’s application runs as a collection of
processes (threads)
• User is concerned about the performance
of the application
• Need to make scheduling decisions based
on process sets
Fair-Share Scheduler
Traditional UNIX Scheduling
• Multilevel feedback using round robin
within each of the priority queues
• If a running process does not block or
complete within 1 second, it is preempted
• Priorities are recomputed once per second
• Base priority divides all processes into
fixed bands of priority levels
Bands
• Decreasing order of priority
– Swapper
– Block I/O device control
– File manipulation
– Character I/O device control
– User processes
Example of Traditional UNIX
Process Scheduling